How to set the container to a fixed width?

Hi, I'd like to set the container to a fixed with of say, 960 pixels (for desktop view obviously), so that resizing ones browser window does not mess up the entire site. The way it is now, the menu bar gets in front of the slider, etc.
How to set the 'fluid' container to a fixed width?
The site in question is http://www.eetcafehetwittepaard.nl

 

Thanks for your reply. Of course I found that setting, but it does not do what I want.
It sets a percentage of the window width. But if you resize the window, the site also resizes.
I want to set a maximum width in pixels, so that the menu in the header stays in the header and does not go in front of the content below.

 

I fixed it in style.css by adding max-width to the body div.